CatDog
by r.b.fanfic
Summary: When a prank of the Marauders has unfortunate consequences. The Black brothers are the one to pay the price. Crack fic. Gift for Tenrousei-kuroi. She was the one who draw the picture and gave me that idea.


Note : Christmas gift for you, Jazz. Your picture is wonderful and gave me so many dreams. I hope you'll like this one I wrote.

The title comes from a cartoon I loved when I was a child.

…

 **CatDog**

Outside his home, it was raining cats and dogs.

A small grimace applied on his face, Regulus Black was looking intently at the piece of clothing he was holding in his hand. A long tear on the front, impossible to miss, still there from his last _battle_ , needed to be fixed for the upcoming meeting with his _Master_. What would the man do indeed if he dared show up with his costume all damaged?

Sighing deeply, Regulus took his wand and tried to remember the spells Kreacher had once taught him.

In the background, a Christmas carol aired and despite the seam he had to do, Regulus couldn't help but smile, stroking absentmindedly the tattoo on his arm.

…

 **Five years earlier**

As far as Sirius Black was concerned, it should **never** have happened. How, in Merlin's name, it had, was beyond his comprehension. But his life, it seemed - and he had learnt it the hard way - was always filled with a series of insignificant events which eventually led to an unbelievable end. Life was unpredictable and for that reason, Sirius couldn't feel bad for what had happened. It was in **no** way his fault.

'Nothing is never your fault' would point out his friends but for once, it was true and he was dealing with the consequences. **The** unpredictable. **The** unbelievable. **The** impossible.

Now that he could think about it, the insignificant events had happened in a particular way, Sirius knew it.

 _First_ , there was Christmas, the last Christmas of the Marauders spent at Hogwarts : an extraordinary moment that begged for an extraordinary action. It had been decided - by whom exactly? Sirius failed to remember, **maybe** it was him actually, after the amount of alcohol they had taken, it was unclear - that prank should be the theme of the holidays.

So, in _second_ , came James Potter and his amazing and yet totally expected idea to go after Slytherins, to go after a special Slytherin, a certain Severus Snape. Since the fallout of Snape and Lily Evan's friendship, James and her had become closer so his best friend had been careful not to do something that would enrage his girl. With his NEWTs and the drool he had to wipe of his chin, he was actually pretty busy. But Lily wasn't here for the holidays and Snape was, as a lot of his suspected Death Eater's companions. The idea of the prank had probably been a bit silly when dealing with dark Wizards and for that part, **maybe** , just **maybe** , it was Sirius' fault but no one could judge him for wanting to stay young.

Which led to the _third_ event. Because of Sirius' history, even if his idea had been unanimously accepted by the group, Remus Lupin had decided he would be the one coming with the spell at the origin of the Transfiguration. It was a safe choice, Remus being the most assiduous student Hogwarts had ever seen. It was only a coincidence that a few minutes before Remus casted, Sirius had reminded him of a movie he had shown them one summer about the special animal Snape would be turned into and **maybe** that had gotten the boy distracted.

But things wouldn't have gone wrong if Peter Pettigrew, as usual overly excited to play a prank on their worst enemy, had let his enthusiasm got the best out of him. **Maybe** Sirius shouldn't have teased him about his clumsiness for the thousandth time that day, but it was irrelevant to the story, really. For as soon as they saw the other boy in the hallway, the rat shrieked too loudly - clumsy even in sounds - which was the _fourth_ event…

… and because of that, Snape saw them, which led to the _fifth_ event. Remus casted and Snape, prepared as he was - **maybe** even more irritated by Sirius' smirk - deflected the transfiguration curse and unfortunately for everyone in the scene, Snape had never lacked power.

Five little events, five insignificant events Sirius had had no part in and a second later, the Transforming Spell fell on someone who wasn't supposed to be here, none other than Regulus Black himself, the boy at Snape's side and then, something went wrong. Something went terribly wrong.

Frankly, it was kind of funny - James, Remus and Peter were laughing after all - but Sirius surely couldn't. Not because his baby brother had just been turned into a cat. Sort of. Let says he was now rather wearing the costume of a cat. But that didn't really matter. It had been a long time since Sirius had had no connection with his brother. He couldn't feel sorry for Regulus. He felt sorry for himself though.

Five little events and a result he had to admit was his fault. Yes, his reaction to Regulus wearing the costume of a cat was entirely Sirius' fault… or more accurately his problem. It shouldn't have been that big of a deal. Sirius' Animagus form wasn't a dog for nothing. But there, things went a bit too far.

Regulus wore a suit usually used by dancers, all black except for the tiny stars incrusted randomly and the white dickey, in a material as shiny as Sirius' leather jacket, with long sleeves, at the end of which a bundle of white fur was gathered, same thing for the leg. On Regulus' head was a hat - what Sirius could only describe as a mix of a hood and a beanie - made of black fur this time, covering all the hair of the boy. And at the top of Regulus' head, at each side, pricked up two pointed ears. Finally, the boy's face was covered with something closed to make-up, giving his eyes, his nose and his mouth the shape of the features of a cat.

What troubled Sirius wasn't any of these features. It wasn't even the tail that came out of Regulus' body, moving in a tantalizing way. He could have lived with a cat-like Regulus from now on. But he couldn't live with a suit so skin-tight.

From the moment he had understood Regulus was a better son than he was, Sirius has ceased to look at his brother, his life being more easiest to live if he did so forgetting he had had someone, at a point, he had deeply loved. So, as Regulus appeared in the hallway, decked out in a very fitting clothing, it was the first time Sirius gave him a real good look. It was the first time he was confronted with the body of his teenager brother, no longer a lanky boy but a man with features both masculine and feminine. It was a shock. An utter shock Sirius would have preferred to avoid.

After all, no one wanted to be reminded of his brother that way. With a costume which made him breathtakingly beautiful, Sirius' mouth suddenly filled with saliva. A brother, for whom, he was actually aroused, so very hard.

…

Regulus was having a bad day. In fact, Regulus was having a bad month, a bad year even. Or maybe he was just having a bad life?

No bad thing could have prepared him to what was actually happening though. He had been following Severus in his walk to the library, sulking because of the dark thoughts flowing into his head when a sudden spell had landed on him. It wasn't a curse he knew and frankly, even if being turned into a cat wasn't something he would have wanted to suffer, it wasn't even the worse thing of the situation. Yes, he was wearing an inconfortable second-skin clothing that made him want to tear it apart. Yes, he was feeling on his face the texture of something unpleasant. But he didn't even care about that. Because at the instant he had raised his head, until then turned to the ground, he had seen his brother. Sirius Black. The boy he never wanted to see again because of his obvious hatred with regard to him. And Regulus couldn't bear his brother's hatred. Not when he had looked up to him his entire childhood. Not when he had wanted to be him. Not when his brother had abandoned him. Not when he was beginning to wonder if his brother had been right all along.

But here he was, in the glory of his physical Black heritage, surrounded by his laughing idiot friends. Sirius didn't seem to be in the mood though, the stare he was giving Regulus was telling.

Annoyed, especially because of the weird atmosphere that had fallen on the hallway, the silence being appropriate, Regulus tried to take off his costume to put an end to this ridiculous mascarade. It didn't work. After a few tries, it appeared the second-skin suit was in fact a second skin, period. Peeling it off didn't work. Neither did the several spells he casted, some dark enough to make the Gryffindors react. But it just didn't work and Regulus was loosing his calm.

« What did you do to me? » he asked, his eyes fierce on Sirius.

His brother didn't answer. He didn't have the time because Potter was here to intervene, always the defender of Sirius, a behavior so disgusting.

« Nothing, » he simply said and Sirius indulged in brother by blushing, something Regulus, and the others too, didn't understand.

« It was just a Transforming Spell, » the half-blood stepped in, « something must have gone wrong. »

« Obviously, Lupin, » sneered Severus as a first contribution to the conversation.

« If I may, » offered the blond boy, raising his wand, and Regulus only nodded briefly, thinking it couldn't get worse.

He just wanted out of the situation. As usual when his brother was involved, he couldn't manage to act like a real Pure-Blood, a real Slytherin, a real Black, which pained him. He wanted to react, he wanted to yell at Sirius, he wanted to confront him, he wanted to ask for his help, but Regulus couldn't allow that. Yes, he wanted to get the hell out of the hallway and fast.

Lupin casted a spell, a second then a third, until his brow knitted funnily. Nothing changed.

« Maybe we should see a Professor, » suggested Severus in a whisper nonetheless heard by the quiet boys.

« Professor McGonagall would be the best » said Lupin easily. « The spell we casted was supposed to be a Transfiguration. I'm coming with you, maybe I can help. »

« Of course, you are, » stated Severus, his pupils darker than ever.

« You'll all come, » concluded Regulus, his eyes still on Sirius. « Especially you, brother. »

Regulus didn't know why he had said that, but one thing was certain, it had made Sirius blush even more, which was definitely weird. It was only fair than the responsible of his state was at least as uncomfortable as he was.

…

This was a nightmare. It had to be. There was no way in hell, it was reality.

In real life, Sirius wouldn't have been able to walk with an erection that big anyway. In real life, he wouldn't have had an erection that big, period. It was nonsense. It was insanity.

Yet, it seemed totally real as their foot led them to the office of McGonagall, as the Transfiguration Professor let them come into her quarters, then listened to the explanations of what had happened. Sirius was quiet. He didn't need to talk. Not that it was a possibility when Regulus' grey eyes - so similar to his own - were on him. He was mesmerized, embarrassed and still very much aroused.

Luckily, not long after their arrival in McGonagall's office, the Professor began to examine his brother, making it possible to breathe again, except his inner dog got jealous of the hand the Director of his house was laying on Regulus. Keeping silent, McGonagall casted several diagnostic spells on the boy and then, when she didn't seem satisfied with the results, she just morphed herself into a cat, the students impressed with her skills. Sirius couldn't help but notice he didn't feel anything towards his teacher but then she was now a real cat. When she began to sniff his brother though, he did feel something. Something strong. Worse than jealousy. He didn't like it one bit.

Then McGonagall got back to her human form and after a last thoughtful look to Regulus, she said, « I think I know what went wrong with the Transfiguration. The problem is that your inner Animagus form is a cat, Mr. Black. How fitting, » she whispered, looking briefly at Sirius, « So I believe you have to complete the transformation before you'll be able to turn back into yourself. »

« But how long will it take? » asked Regulus, disgusting drops of sweat dripping under his suit.

« Usually, you should hold the leaf of a mandrake in your mouth for an entire month before your training could begin properly. But you're already halfway there since you know your form so it shouldn't take that long. »

« More than a month, » whispered Regulus in horror.

« Don't worry, I'll train you, » she tried to reassure the young Black. « In the meantime, boys, » she said turning to the Gryffindors, « let's discuss the schedule of your future detentions. »

…

That's exactly how Regulus and Sirius' life became an ordeal.

Day and night, Regulus living in the same suit, washing himself with magic, relieving himself that way too.

Each moment of the day he spent in the common room or near his brother, Sirius getting aroused, ashamed of himself, feeling like a true Black more than ever.

…

The first few days of Regulus' training went well and thanks to the holidays, the Professor had a lot of time to give him. He didn't want to but he eventually got used to his new form, even if his tail was sometimes difficult to control. Yet, despite McGonagall's encouragement he knew deep down that something was missing to allow him to achieve the full transformation. Nevertheless, Regulus devoted himself to his training, as if it was another class he was meant to pass for his future NEWTs. Usually alone, except for the moments he was at Severus' side, Regulus had no difficulties to escape the mocking the few students at school were throwing at him.

On the 31st of December, an almost sad look on his face, Severus went to find him, in the corner of the library that was as good as home. For a few minutes, Severus said nothing to his friend and Regulus let him keep quiet, perfectly knowing the other boy would speak when he'd feel like it.

« The Dark Lord has heard of your misfortune, » whispered Severus suddenly, after he had casted several Muffliato Charms, « and he can't accept you to any meeting until your appearance is fixed. »

« Oh really, » sneered Regulus, even if he wasn't that angry with the decision of his Master. Severus was the only one with whom he was able to show his real feelings about the situation.

« But he told me that if you still wanted to help him, he was more than willing to accept your house-elf to go on a mission for him. »

« Because I have the choice in the matter? » asked the young Black, mildly amused.

« Regulus! Don't act difficult, please, » warned Severus but the only answer he got was a look of pure despair and he couldn't help but feel bad for his friend, the way he had been feeling since the day Regulus had been turned to a cat. « I know someone who is an Animagus, besides McGonagall, someone who could help you. Maybe you should go see him. »

And then he stared at the floor, making Regulus all the more interested.

« Who is it, Severus? »

…

It was the worse Christmas holidays of Sirius' life. Far worse than any he had spent at Grimmauld Place, which was saying something. Because of his reaction to his brother cat-like form, Sirius had become withdrawn. He didn't want to leave his room actually and wasn't in the mood for any celebration. His friends were worried and at first, they had thought he was angry with them but he had managed to reassure them. He was trying his best to act like his older self but each new encounter with his brother, even if they were in the Great Hall so far away from each other, was leaving him confused and angry with both his hormones and his blackish traits. His reaction had to do with the flaws of his family, incest was after all pretty common among Blacks, one of the many reasons Sirius had done everything he could to flee his family and the sweet words his blood was whispering in his ears.

But it seemed his running had finally come to an end. Sirius didn't know what to do with his new-found desire. Except avoiding his brother and avoiding his sleep - after he had noticed his craving could take full control of his body when his mind wasn't there anymore.

Sirius was almost determined to leave Hogwarts before his graduation but the few times he had tried to cross the gates of the school, he hadn't come down to it.

And so, on the 1st of January, day supposed to be full of new resolutions, Sirius had the mischance to see his brother coming to him, a look of bitterness on his face he didn't want to understand. Unable to compose himself, Sirius didn't even say hello to Regulus. He went the other way, running, just after he heard his brother whisper in wonder :

« Why aren't you laughing? Why do you seem scared? »

In the privacy of his bedroom, Sirius made a promise to himself : he would never let Regulus speak to him again, things wouldn't go further. He was the stubborn Gryffondor Regulus wasn't supposed to be. And to avoid any new unfortunate encounter, he used the Marauders Map. If he didn't see Regulus again, then nothing could happen.

Except his brother was intelligent and despite his wish, very obstinate. It wasn't that much a surprise that Regulus showed up as he was serving his detention in the classroom of Minerva McGonagall under the pretense of working on his Animagus form.

At the very moment Regulus came into the room, Sirius stilled, unable to move, unable to breathe. So attuned to his brother, his dog was now able to smell him before his eyes saw his presence. But then, Regulus moved to the table right next his and Sirius completely lost his mind.

One good look at Regulus, and once again, Sirius was completely aroused, his whole body aware of his brother's own, revealed by his wonderful suit. Luckily, Regulus was sitting at his desk, which prevented Sirius to stare unashamedly at his fitting ass - because, yes, that's exactly what he wanted to do.

After a few minutes spent in silence, Sirius was beginning to wonder if, on top of his craziness, he was also paranoid. Regulus seemed to work his magic and so he managed to relax. A bit. Enough to spare too many glances at his brother.

Then, suddenly, Sirius felt the air around him charged with magic, used to muffle the sounds and conceal their sight.

« I don't want to talk to you that badly I'm gonna run all over the castle to finally be able to, » whispered the voice of Regulus. « But please, listen to me, Sirius. Just one time. Just today. »

Uneasy, Sirius nodded to his brother, who, reluctantly had come for help. It was the least he could do.

« Severus told me you are an Animagus, » continued Regulus, « and you don't need to deny it. He explained to me the situation that led to his acknowledgment. So, you're a dog. »

Again, Sirius only nodded to Regulus as he was becoming more and more uncertain. Taken by his speech, his brother was leaning more and more on him, making it difficult not to do something drastically stupid. As in, tasting him just to appease his inner dog. The dog Regulus was rightfully talking about.

« I need your help, Sirius, or else, I'll never be able to take off that suit. »

And right on cue, Regulus' body made contact with Sirius' own and the eldest of the Black couldn't help but groan both in pleasure of the touch and in disapproval of Reg's want. Removing that fabulous suit was the last thing Sirius desired.

Sirius would have never been able to prevent what happened next. First because Regulus was too clever and he had seen something in Sirius' eyes and in the line of his body he had to understand. Second, because he was far too gone not to accept Regulus' touch on his flesh.

There was the hand of Regulus on his thigh moving and then making contact with his unmissable erection. A simple caress and Sirius had to close his eyes to remember where he was and who he was and what he couldn't do.

But then the hand was gone, his eyes opened again and fell on Regulus' pupils, full of astonishment and wonder. His were probably filled with lust. An unmistakable lust.

« Is that why you run away from me, » mused Regulus but he didn't wait for an answer, he started smirking. « How fitting… for you who despises me. Is that the reason behind your hatred? »

« It has nothing to do with you, » exclaimed automatically Sirius, livid and furious. « Only hormones. Only smell. It's just my dog. »

But deep down, he knew he was lying. He was able to admit it to himself.

« I don't care what caused this, » dismissed Regulus in a small gesture of his head. « Your dog, as you call it, is going to help me. »

« You understand why I can't help you, » said Sirius, frowning.

« I don't care if you get hard when you see me, dressed as a cat, » replied Regulus in a cold voice. « Stop being a fucking Gryffindor. Stop being a pussy and be a man. Be a Black and control yourself, »

« I've never been a good Black, as you perfectly know it, » muttered Sirius as he was feeling both his anger and his desire heightening.

« Which reminds me… You're not a Black anymore. They burnt you from the tapestry. So you're not my brother anymore. It really isn't a problem you get turned on by me. Go indulge in your little desire all you want, but you'll help me, it is settled. »

« You're so like them » spat Sirius with rage, « You disgust me. »

« Well, the bulge in your pant says otherwise, » and on this sentence, Regulus leant his hand forward, squeezed the aforesaid bulge and then stood in a row.

Speechless, Sirius didn't have the opportunity to tell his brother he wasn't agreeing to his mad plan.

That night, hidden behind the canopy of his bed, Sirius jerked off to the pictures of Regulus their brief encounter had given him, and afterwards, he wasn't even feeling that sick.

…

Before his arrival in McGonagall's classroom, Regulus had been uncertain. Now, he was just… even more. Things hadn't gone as planned. They had been better. But Regulus didn't know where that put him exactly.

His first meeting with Sirius showed his brother was very much uncomfortable with the agreement they had made. But if, despite his difficulties, he had managed to ask for Sirius' help, it was only fair his brother had to disregard his own issues. Issues that weren't causing a problem to Regulus. But the young Black didn't really want to think about that. He had thought enough of that issue in the past, in a different way, but still… He had come to terms with his unusual way to see life. Sirius would just have to deal with it.

So he had a problem at hand and he was going to do everything to get rid of it, in the most satisfactory way, no matter what Sirius wanted.

For two weeks, in a very tense atmosphere, the two brothers worked on Regulus' animal form. Sirius had finally agreed to do it until Regulus would be able to turn fully into a cat, even if he had recognized it wasn't his deep desire to let that happen. Each time they saw each other, every day, for a few hours, Sirius did his best to help his brother, staying at the same time away from him. He never touched him. He never did anything that could force them to be close. But at the same time, they managed to learn a bit more about each other. They learnt to get used to each other again. They learnt to like each other again. They learnt to coexist again. Regulus was glad and in the grey eyes of Sirius he could see his brother was too, even if he would never admit it.

Regulus was happy that this part of his life for which he had needed closure for so long was getting better. But something was still blocking his transformation. Until, one evening, as they were working in an abandoned classroom, Regulus had the idea that would solve his problem.

« Maybe you should transform before me, » he offered, desperately sighing because of his last attempt which had been a miserable failure.

« That's not a good idea, » answered Sirius in a shudder. « What do you think will happen if I already have difficulties to control myself when I'm in a human form. I know I'm still me when I become an Animagus, but as an animal, things are less complicated. And if I saw you as a potential partner, I don't even want to know what the dog would do to you. »

« Well, see it that way, Sirius, » countered Regulus who wasn't going to abandon his idea just like that. « Maybe that'll do the same things it did to you when you saw me as a cat. We'll be even, then. And besides, you know my power. I'd be able to get away from you if you attacked me. »

Sirius was about to tell Regulus he was disgusting - again - the young Black saw it in the hard flicker of his pupils but, for an unknown reason, Sirius finally decided to do it.

A second was all it take to say goodbye to the human and welcome to the dog. Suddenly, Regulus had before his eyes a large black dog, with a shiny fur, similar to the Grim. Regulus felt the pull right away, that thing he was missing, but he fought it for a bit, certain it wouldn't be a good idea if he turned for the first time Sirius still in his doggy form. So he waited for his brother to take back his human form and when it was done, he whispered in a funny pant:

« That didn't change a thing… »

« You're not really a cat anyway, » cut Sirius, as if he wanted to justify himself « and here, I was a full dog. »

But Regulus wasn't done talking, despite the increasing of his pants. He needed to turn and quickly.

« That didn't change a thing, Sirius, because I don't need a costume or a transformation to recognize you're more than handsome enough to be easily used as a wank material. »

And before Sirius could react, Regulus finally made his first transformation into a large cat with a black and white fur.

And then he ran, ready to become himself again.

…

If Sirius had believed a normal Regulus would mean the return of his normal life, he quickly understood he was wrong. Things were worse.

As if the wrong transfiguration had awakened something in him, he wasn't able to turn if off. The reactions weren't as quick as before, when Regulus had worn the costume of the cat, but he was still getting turned on by his brother. Now, he didn't think himself insane, he just wanted answers.

Answers to the questions Regulus' behavior on their last meeting had raised. He hadn't talked to Regulus since that fateful night and each hour had been a torture, a constant questioning of what Regulus had meant. Did his brother have recognized he, too, fantasized about him? That couldn't be. And even if it was, it was definitely wrong. But Sirius couldn't help but wonder.

What if?

More than making him wonder, the new attitude of his brother, 'Elusive Regulus' as he had taken to call him in his head, was worrying him. For a reason he couldn't explain, Regulus' features had begun to pale since his return to his full human form. And even if Sirius had sworn one day he would never concern himself with his family again, it wasn't something he could avoid. In his head, he had given Regulus until the 25th of the month to come speak to him. If he didn't do it, he was going to go after him and things were going to be said. Things were going to be done.

It was the 24th of January, exactly one month after the prank that had gone badly. At 8.55am the moon was expected to be full, and with that, Remus was turning into a wolf. Sirius was actually happy to miss a day of school to turn into Padfoot, during which he would be able to lose all the weigh of the stress the situation with his brother was bringing on his shoulders.

As usual, the four Marauders had met a few hours before the fateful moment, near the Forbidden Forest. The turning went well and few minutes later, they were in the Forest, enjoying themselves.

It had happened before that a smell was strong enough to bring Sirius off his path of distracting the wolf for half a second. Never a smell had been so intense it made him completely forget about his friends though. But as he was running in the Forest, happy for the first time in a long while, that's exactly what happened.

The smell was strong, powerful, heady, intoxicating and Sirius had only one urge, to follow it until he could find its source. The source was easy to chase, as if every other smell, every other color, everything that could have been a distraction, had been turned off.

Only the odor of his prey mattered.

It shouldn't have come as a surprise that his prey was a cat, a large cat he immediately recognized as his brother. As he came closer to him, unable to remember the reason why he shouldn't, they began to communicate, as he was able to do with his friends in a telepathic sort of way when they were Animagi.

 _Why are you here?_ asked the dog.

 _You weren't at breakfast,_ answered the cat. _I was… worried._

In Animagus form, the nuances of the voice weren't as easily read. Was it irony or uncertainty Regulus felt?

 _How did you find me?_ howled Padfoot.

 _I morph myself into my cat and I found you thanks to your smell._

Was it mockery?

 _You smell good_ , the dog moaned, unable to help himself.

 _You too._

This time, the meaning behind the words was obvious, because of the mewing that followed. Regulus wanted this. Following his nose, quite literally, Padfoot approached the cat until they were finally touching.

First, their noses bumped, both humid, heady with the odor they perceived.

Second, their heads stroked each other, leaving behind their mark, making the other want to act.

Of course, it was Padfoot who acted first, always a Gryffindor even when Sirius' mind was shut down. His tongue, enthralled by the smell of the cat, wanted to taste, and soon it was on Reg's neck and he began to lick diligently. Along the way, Padfoot moved behind the cat. Now completely wet with his licks, he was meowing more and more, giving Padfoot a desire to mate too strong to be undermined, even by nature.

His forelegs holding the cat's body, the dog began to rub himself against his backside, the tail of the cat providentially raised to welcome him. Getting into a rut wasn't something he could stop.

Sirius didn't even try, as he kept licking the neck of Reg, each time he could, until the cat completely got into position, his forelegs entirely folded.

From time to time during the mating dance, Sirius' mind was coming back and even if he was aware, deep down, he would come to regret what he was doing, nothing seemed strong enough to stop him.

Except there was something. The special mark every Animagus had on their body, something unique, you couldn't forge, you didn't chose. And it's in their new position, Sirius suddenly saw the remnants of the Dark Mark. Immediately, Sirius took hold of Padfoot and he threw the cat away from him, his brother, and in one thought he morphed into his human form again.

Regulus wasn't long to follow and as if what had just happened was totally normal, he started the conversation, untroubled with the two big revelations the meeting had given them.

« That's one of the thing I wanted to talk to you about, » he said, in a calm voice, as if, one minute ago, he hadn't unashamedly offered his hole to his own brother.

« You're one of them, » stuttered Sirius who hadn't wanted to believe it until then - not his baby brother.

Regulus took a step closer and in this gesture, Sirius saw the depths of his vulnerability but he was too shocked to care for the moment.

« Tell me everything, » he offered, « I'll listen to you. »

And that's exactly what Regulus did. He told Sirius the story of his life, from his point of view, explaining to his brother what he had done just to survive. He told him how much he had worshipped Voldemort, especially after Sirius had left their house, he told him how badly he had been disappointed once he had met the man, once it was too late, as he had taken the Mark. But his change of heart hadn't really come until a few days ago, when Voldemort had required the use of Kreacher on a mission. It had coincided with the end of his ordeal as a semi cat and luckily, or otherwise the house-elf would have died. Regulus finally told Sirius he had had to find a new elf for his parents, as Kreacher was resting because of the poison Voldemort had made him take. And it was all. It was already a lot, especially coming from a Slytherin, but Sirius wasn't satisfied.

« So you only regretted your decision because of your elf-friend? » he sneered.

« That's not exactly what I just told you, » Regulus answered, rolling his eyes. « But that's what made me want to act, yes. I know the Dark Lord is dangerous and what he wants as nothing to do with Pureblood. He wants immortality and that's just sick. »

« You really don't want to do this because it's the moral thing to do, » muttered Sirius as if he couldn't believe it.

« I'm not a moral person, Sirius. And neither are you. You know it. I know it. We're alone so you don't have to play an act with me. »

Furious, particularly because he knew Regulus was right, Sirius howled into the wood, as if he was still a dog. Even if he had been a moral person before - which wasn't the case, he was more Black than he wished to admit - he wasn't anymore. Not since he had begun to act on his attraction to his own brother in the privacy of his room, not since right now as he had wanted to mate with him and hadn't stopped himself while he could have.

« So, are you going to help me? » asked Regulus after a few minutes of silence.

« What do you want to do exactly? »

« I want to go in the cave the Dark Lord took Kreacher, find the objet he put there, and give it to the Order of the Phenix. After that, I don't know. »

« Alright, then, »

« I suppose you'll want to involve your friends… » assumed Regulus, trying to speak casually when his voice was shaking with relief.

« No, I don't. »

It was something he had to do with his brother only, something that would determine their future.

…

Being alone with Sirius was easy now. Since the moment they had almost mate in the Forbidden Forest, since the moment Sirius had agreed to help him, Regulus had noticed something had clicked in Sirius' mind. He wasn't fighting anymore. He wasn't fighting their closeness. He wasn't fighting his Black personality. He was just waiting… for something.

As they were planning their escapade in the Cave of the Dark Lord, Regulus and his brother hadn't morphed into their Animagus form to avoid a repetition of the last time. Not because they didn't want it, but because they needed to focus on their mission.

So, when they entered the Cave, accompanied by Kreacher and they morphed themselves into their animal, the pull to be closer was a hard thing to fight. But they managed because they were dedicated, they had always been in their own way.

Thanks to their animal form, the Dark Lord's protections of the Cave did nothing to them. As he had understood it once Kreacher had recovered from the poison he had drunk, Regulus knew the Dark Lord was only afraid of Wizards and didn't think creatures could really harm him. It was his mistake.

Coming into the Cave, then finding the small boat, and finally arriving on the island surrounded by the waters full of Inferi was a blur in Regulus' mind. All he remembered was that at a point, he had decided he would be the one lapping the poison of the bowl, just because, one day when he was young, Sirius had told him you could know what a man was like by looking at how he treated his inferiors. As a matter of fact, Regulus considered Kreacher like his equal and he couldn't fathom the thought of his house-elf drinking again that vile potion. So he did and awful memories, awful nightmares appeared in his head, a lot of them involving Sirius.

But, in real life, Sirius was at his side, Padfoot was at his side, licking his fur, not to show him he wanted to mate, just to comfort him and it was enough for Regulus.

He felt sick, felt like he was going to die, but he knew his Animagus form was helping him in a way his human body couldn't have. And so he faced what was coming.

They retrieved the object at the end of the basin - what they'll understand later as being a Horcrux - and then Kreacher Apparated them directly next to Dumbledore's office, as they had planned before.

For a while, Regulus passed out and when he finally awakened, he found himself in the infirmary, in the arms of his brother. Madam Pomfrey had managed to cure him, explained Sirius, and even if his brother hadn't been needed anymore, he hadn't wanted to move. He wanted to be present for Regulus' waking.

And here he was.

« What now? » asked Regulus in a small voice.

« Now, Dumbledore is taking care of the Horcrux with the Order. So, thanks to you, there is a good chance Voldemort will be eradicated of the Earth in a few days, weeks or months… This is not our business for now. We're school boys after all. »

« That's not what I meant. »

« What did you mean then, Regulus? »

« What now, about us? »

« What do you want? »

« You, » replied Regulus, immediately, sure of himself.

« The dog? » asked Sirius, uncertain.

« All of you. »

And then his brother's lips were on his.

« Are you done fighting? » moaned Regulus into Sirius' mouth, his breath taken away by his brother's tongue.

« I am, » was the only answer he got before Sirius started another kiss, the beginning of the mating dance they wanted to do for too long.

All naked in the arms of Sirius, his brother deep inside him for the first time, the first push, Regulus couldn't help but meow, aware he wasn't supposed to be a cat at that instant. But Sirius, all of Sirius, from his smell to his passion, was calling to his inner animal. And the moment the sound crossed his lips, Sirius let go of the tenderness he had wanted to show Regulus.

His brother instincts prevailed and Sirius turned Regulus on his stomach getting him on his four, taking him doggy style, the way how things should be. Sirius' arms tightening around his waist, Regulus felt whole, complete and the bite his brother was giving to his neck proved he was feeling the same.

« You're mine, » howled Sirius and Regulus couldn't help but meow in agreement. « I want you so hard, both a boy and a cat. »

And before Regulus could answer, Sirius began to pound in earnest. Only after his brother's seed was dripping from his hole and his own was drying on his stomach and the sheets did he manage to get back his Slytherin verve.

« If you're a good boy, a good dog, maybe you'll have me with that suit you so adore. »

And Sirius moaned, « I praise the day that spell got wrong, it gave me everything I wanted. Life always find a way to show you the deepest desire of your heart, and despite the unpredictable of the situation, I have to admit, it was kind of my fault, **maybe** entirely my fault… and certainly my luck. »

As Sirius began to laugh about his own idiocy in the affair, Regulus just showed him his neck, turning his head to the side to give him a good look. Sirius instantly stopped laughing as Regulus had intended. His mouth found his brother's neck and he gave a tentative lap, the young Black received with happiness.

« Do you want to try in Animagus form? »

…

 **Five years later**

As usual since the moving in their house, Sirius and Regulus' Christmas Eve party had been a success. All their friends had been there, friends that in another life would have never managed to attend a diner together, even less a celebration so important. But, with the extermination of Voldemort, four years earlier, lots of things had changed. As the status of Sirius and Regulus' relationship. They were no longer brothers. That role had been completely forgotten during their first night of passion and the several others that had followed.

Their friends were clever but they never asked any question, even if they all knew something had changed when Regulus had turned into a cat. Sirius was glad they didn't question them. They accepted them all the same. Like he was accepting the agreement James, Lily and Severus had found in their relationship, like he was accepting Peter and Mary complicated love story, like he was accepting Remus' forced single life. Nobody judged, it was perfect.

What was really perfect though was coming after the party, with everyone gone except for him and his brother. Christmas was after all particular for the two brothers and as if it was the real anniversary of their relationship's beginning, they alway celebrated in a special way.

Right now, Sirius knew, Regulus was waiting for him in their Master bedroom, wearing the suit that had started everything between them. A costume he usually abused in his eagerness to see Regulus naked. All naked for him. With only his mark on him, the tattoo they both wore : a dog and a cat intimately entwined. The Dark Mark had been long gone with the death of Voldemort, and both had been able to show the world their ownership, their true allegiance, in the form of the tattoo. Sirius remembered with a certain fondness that memorable day…

But his reminiscent quickly came to an end.

A smile on his face, Sirius began to take off the suit he had been wearing for the night. There was no time to lose, if the meowing Regulus had just let out meant anything. Sirius barked as an answer, joyful and aroused. He couldn't wait to be inside Regulus.

Outside, it was raining cats and dogs. Again.

Fin.


End file.
